The present invention relates to an ultrasonic Doppler blood flowmeter which can carry out imaging using ultrasonic waves, and which can simultaneously measure the speed of a moving fluid based upon the pulse Doppler method.
In recent years, an ultrasonic Doppler blood flowmeter has been employed which can display a sonagram based upon the Doppler effect and a tomogram due to ultrasonic waves at the same time and in real time by using both of the ultrasonic pulse Doppler method and the ultrasonic pulse reflection method. This ultrasonic Doppler blood flowmeter has been widely used for diagnosing the cardiovascular system and organs of a living body. The construction of the above ultrasonic Doppler blood flowmeter is described in, for example, Japanese patent application JP-A-sho 55-54,941. This conventional ultrasonic Doppler blood flowmeter will be explained below, with reference to FIG. 1.
FIG. 1 is a block diagram for explaining the basic principle of the conventional ultrasonic Doppler blood flowmeter. In FIG. 1, reference numeral 90 designates a probe, 91 designates a transmitting/scanning circuit, 92 designates a receiving/scanning circuit, 93 designates a phase detector, 94 designates a frequency analyzer, 95 designates an amplitude detector, 96 designates a display device, and 97 designates a controller.
Next, the operation of the above blood flowmeter will be explained. A drive pulse, which is generated by the transmitting/scanning circuit 91, is applied to the probe 90, to transmit an ultrasonic wave in directions m.sub.1, m.sub.2 , . . . and m.sub.n. The receiving/scanning circuit 92 is controlled so as to have high sensitivity in the directions m.sub.1, m.sub.2, . . . and m.sub.n. In the Doppler mode, ultrasonic waves are repeatedly transmitted and received in a specified direction, for example, in a direction m.sub.d. The received signal is subjected to phase detection, to obtain I- and Q-signals which indicate Doppler shift data. The I- and Q-signals are subjected to frequency analysis, to determine the moving speed of blood in a body that is being inspected, and the speed is displayed on the display screen of the display device 96. In the B-mode, the transmission and reception of ultrasonic waves are carried out successively in the directions m.sub.1, m.sub.2 and so. The received signal thus obtained is subjected to envelope detection by the amplitude detector 95, to display a tomogram on the display screen of the display device 96. Either the Doppler mode or the B-mode is selected by the controller 97. In order to obtain information on the flow of blood in a sampling volume S existing in the direction m.sub.d and a tomogram due to the B-mode at the same time, the transmission and reception of ultrasonic waves in the directions m.sub.1, m.sub.2, . . . and m.sub.n due to the B-mode and the transmission and reception of ultrasonic waves in the direction m.sub.d due to the Doppler mode are alternately carried out on the basis of a command from the controller 97. This operation mode will hereinafter be referred to as "B/D mode".
When the repetition frequency of ultrasonic pulses in the Doppler mode is expressed by fr, the repetition frequency of Doppler operation in the B/D mode may be reduced to fr/2. Thus, there arises the problem that the maximum measurable blood speed is reduced to the half of the maximum measurable blood speed in the Doppler mode.
In order to solve this problem, a method has been devised which can obtain a tomogram due to the B-mode, without reducing the sampling frequency of Doppler data. An example of this method is described in Japanese patent application JP-A-sho 61-25,534. According to this method, the transmission and reception of ultrasonic waves in the Doppler mode are repeated three times and then the transmission and reception of ultrasonic waves in the B-mode are once carried out by using, for example, the blood flowmeter of FIG. 1. The output of the phase detector 93, which is not generated during the period when the flowmeter is operated in the B-mode, is calculated by an interpolation method. Thus, the sampling frequency of Doppler shift data is not reduced.
The interpolation method in this case, however, has been devised to process a conversational or voice signal, and is not suited to produce supplementary data for a signal having a frequency component corresponding to half of the sampling frequency, such as Doppler shift data according to the above operation. FIG. 2 shows the results of the above interpolation method for Doppler shift data which is obtained by transmitting ultrasonic pulses twice in the Doppler mode and by transmitting an ultrasonic pulse once in the B-mode, and thus has a frequency component corresponding to half of the repetition frequency of the transmitted pulses. As shown in FIG. 2, a supplementary value obtained by the interpolation method is entirely different from the expected value. In other words, a frequency which is obtained on the basis of the supplementary value is shifted to the low frequency side. Thus, there arises the problem that the frequency obtained by the frequency analyzer is different from the correct frequency.